Preheat oven to 175C and line two baking trays with baking paper and set aside. Put butter sugar and vanilla in a clean bowl and beat with electric beaters until pale and fluffy. Add egg and, once combined, fold in flours and zest. Roll dough into walnut-sized balls and place on baking tray about 5cm apart. Pop into oven and bake for 12-15 minutes or until top springs back. Remove from oven and allow to cool on tray.

To make icing, mix together icing sugar, butter and milk. Icing should have a firm but runny consistence. Once biscuits have cooled, ice generously, then cover with coloured sugar sprinkles. Allow the icing to set before serving.